May 10 (Reuters) - Merck & Co said on Wednesday U.S. health regulators approved its Keytruda in combination with chemotherapy for previously untreated advanced lung cancer, solidifying the drugmaker’s lead position in the field of medicines that help the immune system fight cancer.

Lung cancer is by far the largest oncology market and the approval significantly expands the number of patients available for Keytruda therapy.

Merck shares were up 3.2 percent at $65.99 in extended trading.

“This is the key game changer for Keytruda,” said Leerink Partners analyst Seamus Fernandez, adding that he expects physician acceptance to be “pretty robust.”

The accelerated approval was based on data from a study of 123 previously untreated patients with metastatic non-squamous non-small cell lung cancer (NSCLC).

“It was a small trial but the results were really quite striking,” said Roy Baynes, head of global clinical development for Merck Research Labs.

The Food and Drug Administration approved Keytruda in combination with Eli Lilly’s Alimta, the chemotherapy drug used in the study that led to the agency’s decision.

Merck may be asked to conduct another trial to confirm the clinical benefit of the combination.

Keytruda alone was already approved as an initial, or first-line, therapy for advanced NSCLC in patients whose cancer cells have a high level of the PD-L1 protein the drug targets. The combination allows for treatment regardless of level of PD-L1.

Merck scored a coup last year, when Keytruda extended patient survival in a first-line lung cancer trial, supplanting Bristol-Myers Squibb as the perceived leader in the field. Bristol’s rival drug Opdivo surprisingly failed to show a survival benefit compared with chemotherapy in a similar study.

Both drugs and one from Roche were already approved for lung cancer once a prior treatment fails or stops working.

The medicines belong to a new class of cancer drugs, called PD-1 or PD-L1 inhibitors, that block a mechanism tumors use to evade detection by the immune system. There are now five approved for a variety of cancers, but all companies involved are aiming for a slice of the lung cancer market.

Bristol-Myers is testing Opdivo with its other immunotherapy, Yervoy, as a first-line NSCLC treatment, but did not seek accelerated approval. AstraZeneca is also testing an all immuno-oncology combination with highly anticipated data expected mid-year.

The original Keytruda first-line approval allowed for treatment of about 30 percent of NSCLC cases. The combination could be used on all patients with non-squamous NSCLC, which accounts for about 75 percent of lung cancer cases. (Reporting by Bill Berkrot; Editing by James Dalgleish)